High temperature structural static seals are used in gas turbine engines and other equipment where high temperature fluids need to be sealed. These seals are traditionally made from cold formable superalloys with superior strength characteristics at elevated temperatures. Conventional seals typically are made from superalloy sheet stock and can have different shaped cross sections. For example, some conventional cross-sections include the C (FIG. 1A), U (FIG. 1B) and E shapes (FIG. 1C), among other annular seals. These conventional annular seal rings are preferably installed between flanges, for example F1, F2 (FIG. 2), in a compressed condition.
The compression stress and the fluid pressure acting on these types of conventional annular seals provide sufficient sealing force to prevent the high pressure fluid from leaking through the interfaces where the seal and the flanges meet. At temperatures less than about 1300° F., as the flanges move back and forth, the elastic spring back of the seal cross-section maintains the sealing contact with the flange, as best illustrated in FIG. 3. However, at high temperatures of about >1300° F. conventional seals made of cold formable superalloys, such as alloy 718 Waspaloy and the like, have been found to stress relax because of coarsening and dissolution of the strengthening phase γ′. At high temperatures, these seals under compression deform permanently to a compressed state, and lose the ability of elastic spring back and sealing contact with the flanges as they move away during the operating cycle of the engine, thus creating a gap, “g” between the seal and the flange. This creates a leak path through which the pressurized fluids can flow, as illustrated schematically in FIG. 3. The spring rate of conventional seals is controlled by the thickness and shape of seal cross-section as well as the yield strength, and elastic modulus of the cold formable alloys from which the seals are made.
In order to avoid permanent aforementioned failure of seal performance resulting from exposures at high temperatures, generally of about >1300° F. or so, it is known to keep the temperature of these seals from reaching such elevations. One manner in which conventional seals are cooled is by using bleed air from compressors of gas turbine engines. Although generally effective, the use of such bleed air is expensive and could be otherwise used for generating thrust or power. The use of cooling air, therefore, adversely affects the efficiency of gas turbine engines. A need exists for cost-effective high temperature structural seals which can maintain their sealing contacts at high temperatures of about >1300° F. without the need of cooling air.
Similarly, a high temperature fastening device, using its spring action and holding two components with widely different thermal expansion coefficients, such as metallic and ceramic components, can lose its fastening capability at high temperatures. For example, lightly loaded annular spring devices are necessary to attach a ceramic liner to a metallic casing of a combustor as shown in FIGS. 4A-4B. The ceramic is not rigidly fastened to the metallic casing using a bolted design because the bolt stresses generated by the differential thermal expansion of the metallic casing and ceramic liner can lead to failure of the brittle ceramic liner. Thus, a need also exists for a low load spring device which can operate at extremely high temperatures of about >1300° F., up to about 1800° F. These applications are generally in oxidizing environments and any such spring devices should also posses oxidation resistance.